Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements

Synthetic-aperture radar imaging of the ocean surface : theoretical considerations, and experiments with simulated and actual SAR imagery

Three key areas of controversy in synthetic-aperture radar (SAR) imaging of ocean surface waves are considered: first, the nature of Bragg scattering; second, the role, magnitude, and calculation of the scene coherence time; and third, the relevant ocean wave velocities for coherent Doppler modulations. This work begins with a re-derivation and extension of existing SAR imaging theory for point and diffuse targets. Generic, relatively simple, closed-form expressions for the impulse response, the resolution, and the image bandwidth summarize this unified treatment. Theoretical differences between the imagery of point and diffuse targets are pointed out. Based upon these fundamental differences, a statistical testing procedure is formulated to address the question of scene target density. Background ocean surface wave theory is outlined in preparation for discussions of SAR ocean imaging. Of central importance is the role of the phase velocity, which is the speed of translation of the mean pattern of reflectivity, and the orbital motion, which leads to coherent (phase) modulation, and hence to velocity bunching, acceleration defocus, and target decorrelation. Based upon this theoretical background, one- and two-dimensional simulation models are developed. The one-dimensional simulation addresses the effects of various parameters upon the mean image contrast in a velocity bunching model and guides the development of the two-dimensional simulation. The two-dimensional simulation is unique because each target which constitutes the scene is explicitly considered. This leads to a degree of control and flexibility which is not available from actual SAR imagery. Qualitative and quantitative comparisons are drawn between the simulated and actual SAR imagery to address the key areas of controversy. The assertion that Bragg scattering is a coherent process is defended, despite inability to conclusively verify this using SEASAT data. Comparisons between simulation and C-SAR imagery of waves propagating into ice verify the roles of the scene coherence time and the wave phase velocity.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat

Validating Targets Detected by SAR Ship Detection Engines

Validating detected targets by ship detection engines without ground-truth data is a very challenging task and this is addressed in this article. Targets detected in synthetic aperture radar (SAR) imagery are validated against automatic identification system (AIS) messages and using visual assessment procedures. Targets detected by the OceanSuite ship detection engine were used as the baseline for validation purposes. OceanSuite has been tuned to minimize the false alarm rate while providing a good overall detection rate. RADARSAT-2 images are often used for ship detection; maritime satellite surveillance radar mode images are considered in this article. An appropriate (spatial and temporal) AIS message is first selected, then the SAR ship signature location is predicted; the predicted ship signature location may be automatically associated with the OceanSuite detections based on proximity. Finally, all detections are validated through visual assessment. Key outcomes of this work are the assessment criteria for declaration as a ship target, tools for automatic validation of SAR detections against AIS-reported targets, and visual assessment results. The RADARSAT-2 images used in this work were collected near Vancouver Island and the Canary Islands. OceanSuite demonstrated a 78% accuracy for AIS-reported targets and over 80% of the validated targets were declared by OceanSuite. Visual assessment showed that a few false targets were declared by the OceanSuite ship detection engine